This invention relates to novel ester compounds useful as monomers to form base resins for use in chemically amplified resist compositions adapted for micropatterning lithography, and a method for preparing the same.
While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using a KrF or ArF excimer laser as the light source is strongly desired to reach the practical level as the micropatterning technique capable of achieving a feature size of 0.3 xcexcm or less.
The resist materials for use in photolithography using light of an excimer laser, especially ArF excimer laser having a wavelength of 193 nm, are, of course, required to have a high transmittance to light of that wavelength. In addition, they are required to have an etching resistance sufficient to allow for film thickness reduction, a high sensitivity sufficient to eliminate any extra burden on the expensive optical material, and especially, a high resolution sufficient to form a precise micropattern. To meet these requirements, it is crucial to develop a base resin having a high transparency, rigidity and reactivity. None of the currently available polymers satisfy all of these requirements. Practically acceptable resist materials are not yet available.
Known high transparency resins include copolymers of acrylic or methacrylic acid derivatives and polymers containing in the backbone an alicyclic compound derived from a norbornene derivative. All these resins are unsatisfactory. For example, copolymers of acrylic or methacrylic acid derivatives are relatively easy to increase reactivity in that highly reactive monomers can be introduced and acid labile units can be increased as desired, but difficult to increase rigidity because of their backbone structure. On the other hand, the polymers containing an alicyclic compound in the backbone have rigidity within the acceptable range, but are less reactive with acid than poly(meth)acrylate because of their backbone structure, and difficult to increase reactivity because of the low freedom of polymerization. Additionally, since the backbone is highly hydrophobic, these polymers are less adherent when applied to substrates. Therefore, some resist compositions which are formulated using these polymers as the base resin fail to withstand etching although they have satisfactory sensitivity and resolution. Some other resist compositions are highly resistant to etching, but have low sensitivity and low resolution below the practically acceptable level.
An object of the invention is to provide a novel ester compound useful as a monomer to form a polymer for use in the formulation of a photoresist composition which exhibits a high reactivity and substrate affinity when processed by photolithography using light with a wavelength of less than 300 nm, especially ArF excimer laser light as the light source. Another object is to provide a method for preparing the ester compound.
The inventor has found that an ester compound of formula (1) can be prepared in high yields by a simple method and that a resist composition comprising a polymer obtained from this ester compound as a base resin is improved in sensitivity, resolution and substrate adhesion.
The invention provides an ester compound of the following general formula (1). 
Herein R1 is hydrogen or a straight, branched or cyclic alkyl group of 1 to 6 carbon atoms, R2 is an acid labile group, k is 0 or 1, and m is an integer from 0 to 5.
Preferably the ester compound has the following general formula (2) or (3). 
Herein m is as defined above, R3 is hydrogen or methyl, R4 to R7 are independently selected from straight, branched or cyclic alkyl groups of 1 to 15 carbon atoms, the sum of carbon atoms in R4, R5 and R6 is at least 4, and Z is a divalent hydrocarbon group of 4 to 15 carbon atoms which forms a ring with the carbon atom to which it is connected at both ends.
A method for preparing the ester compound forms another aspect of the invention, which involves the step of effecting addition reaction of a metal enolate of acetate of the following formula (5) to a carbonyl compound of the following formula (4). 
Herein k, m, R1 and R2 are as defined above, M is Li, Na, K, MgY or ZnY, and Y is a halogen atom.
The ester compounds of the invention are of the following general formula (1). 
Herein R1 is hydrogen or a straight, branched or cyclic alkyl group of 1 to 6 carbon atoms. Exemplary alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, tert-amyl, pentyl, hexyl, cyclopentyl, and cyclohexyl. R2 is an acid labile group. The letter k is 0 or 1, and m is an integer from 0 to 5 (i.e., 0xe2x89xa6mxe2x89xa65), and preferably from 0 to 3.
The preferred acid labile group represented by R2 are those of the following formulas. 
R4 to R7 and Z are as defined below.
Preferred among the ester compounds of formula (1) are ester compounds of the following general formula (2) or (3). 
Herein m is as defined above. R3 is hydrogen or methyl. R4 to R7 are independently selected from straight, branched or cyclic alkyl groups of 1 to 15 carbon atoms. The total number of carbon atoms in R4, R5 and R6 is at least 4. Examples of the straight, branched or cyclic alkyl groups of 1 to 15 carbon atoms include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, tert-amyl, pentyl, hexyl, cyclopentyl, cyclohexyl, cyclopentylmethyl, cyclopentylethyl, cyclohexylmethyl, cyclohexylethyl, bicyclo[2.2.1]heptyl, bicyclo[2.2.2]octyl, bicyclo[3.3.1]nonyl, bicyclo[4.4.0]decanyl, tricyclo[5.2.1.02,6]decanyl, tetracyclo[4.4.0.12,5.17,10]dodecanyl, and adamantyl. Z stands for divalent hydrocarbon groups of 4 to 15 carbon atoms, such as alkylene and alkenylene groups, which forms a ring with the carbon atom to which it is connected at both ends. Examples of the rings that Z forms include cyclopentane, cyclopentene, cyclohexane, cyclohexene, bicyclo[2.2.1]heptane, bicyclo[4.4.0]decane, tricyclo[5.2.1.02,6]decane, tetracyclo[4.4.02,5.17,10]dodecane, and adamantane.
Illustrative, non-limiting, examples of the ester compounds of formula (1) and formulas (2) and (3) are given below. 
As seen from the reaction scheme shown below, the ester compound of formula (1) can be prepared by the step of causing a base to act on a corresponding acetate of formula (6) (where X is hydrogen) or a corresponding haloacetate of formula (6) (where X is halogen) to form a metal enolate of formula (5) and effecting nucleophilic addition reaction of the metal enolate to a carbonyl compound of formula (4). 
Herein, k, m, R1 and R2 are as defined above. X is hydrogen or halogen. M is Li, Na, K, MgY or ZnY, and Y is halogen.
The bases used for forming the metal enolate include metal amides such as sodium amide, potassium amide, lithium diisopropylamide, potassium diisopropylamide, lithium dicyclohexylamide, potassium dicyclohexylamide, lithium 2,2,6,6-tetramethylpiperidine, lithium bistrimethylsilylamide, sodium bistrimethylsilylamide, potassium bistrimethylsilylamide, lithium isopropyl-cyclohexylamide, and bromomagnesium diisopropylamide; alkoxides such as sodium methoxide, sodium ethoxide, lithium methoxide, lithium ethoxide, lithium tert-butoxide, and potassium tert-butoxide; inorganic hydroxides such as sodium hydroxide, lithium hydroxide, potassium hydroxide, barium hydroxide, and tetra-n-butylammonium hydroxide; inorganic carbonates such as sodium carbonate, sodium hydrogen carbonate, lithium carbonate and potassium carbonate; metal hydrides such as boranes, alkylboranes, sodium hydride, lithium hydride, potassium hydride, and calcium hydride; alkyl metal compounds such as trityl lithium, trityl sodium, trityl potassium, methyl lithium, phenyl lithium, sec-butyl lithium, tert-butyl lithium, and ethyl magnesium bromide; and metals such as lithium, sodium, potassium, magnesium, and zinc, but are not limited thereto. It is noted that reaction using haloacetate and zinc is known as Reformatsky reaction.
In the addition reaction of the carbonyl compound of formula (4) with the metal enolate of formula (5), 0.8 to 1.5 mol of the metal enolate is preferably used per mol of the carbonyl compound. Useful solvents are ethers such as tetrahydrofuran, diethyl ether, di-n-butyl ether, 1,4-dioxane, ethylene glycol dimethyl ether, and ethylene glycol diethyl ether and hydrocarbons such as hexane, heptane, benzene, toluene, xylene and cumene, alone or in admixture thereof. The reaction temperature and time vary with particular starting reactants used. In one example where an acetate of formula (6) wherein X is hydrogen and a strong base such as lithium diisopropylamide or lithium bistrimethylsilylamide are used, the preferred reaction conditions include a reaction temperature in the low range of xe2x88x9280xc2x0 C. to xe2x88x9230xc2x0 C. and a reaction time of about xc2xd to 3 hours because the metal enolate is thermally unstable. In another example where a haloacetate of formula (6) wherein X is halogen and a metal such as zinc or magnesium are used, it is generally preferred to keep the reaction temperature in the range of 20 to 80xc2x0 C. and the reaction time in the range of about 1 to 20 hours. The reaction conditions are not limited to these ranges.
A polymer is prepared using the inventive ester compound as a monomer. The method is generally by mixing the monomer with a solvent, adding a catalyst or polymerization initiator, and effecting polymerization reaction while heating or cooling the system if necessary. This polymerization reaction can be effected in a conventional way.
A resist composition is formulated using as a base resin the polymer resulting from polymerization of the ester compound. Usually, the resist composition is formulated by adding an organic solvent and a photoacid generator to the polymer and if necessary, further adding a crosslinker, a basic compound, a dissolution inhibitor and other additives. Preparation of the resist composition can be effected in a conventional way.
The resist composition formulated using the polymer resulting from polymerization of the inventive ester compound lends itself to micropatterning with electron beams or deep-UV rays since it is sensitive to high-energy radiation and has excellent adhesion to substrates, sensitivity, resolution, and etching resistance. Especially because of the minimized absorption at the exposure wavelength of an ArF or KrF excimer laser, a finely defined pattern having sidewalls perpendicular to the substrate can easily be formed. The resist composition is thus suitable as micropatterning material for VLSI fabrication.